
Also this is the age wehn males are trying to assert themselves and find their place in the grand scheme of things.
They are a natural guard and will do so whenever they perceive a threat. A threat will normally be anything out of the ordinary and anyone exhibiting strange behaviour, this will include fear and wariness from other people (which many will display simply because it is a large guarding breed), especailly when on lead.
Many young adult or mature male dobes are not renowned for their good behaviour around other assertive male dogs.
The best approach is to keep the dog focussed on you and demand obedience (tell him to sit) and quietly praise for leaving the person dg or what have you. You must not get cross or uptight as this will confirm in his mind that he had reason to be wary/on guard as you are upset.
Basically your manner should tell him "Oi, I don't need looking after, and if there is any growling biting to be done then I will do it"
I walk every day with my freind and her baby and a 2 1/2 year old male and he will often respond suspiciously to strange behving people. Especially young men who for some reason think it appropriate to eye ball him! He gets excited when eh sees another dog too.
His owner gets his attention and insists he leaves and walks quietly on, or that he sits quietly until the thing that has got him going has passed.
He is greatoff lead and totally relaxed as if he fels off duty. His only interest in life then is his ball. He is fine with other dog off lead too, and would rather fetch his ball.